Zem kneels upon the brink, and squints at the rotating treasures to time his grab at the hammer just so...

Too late Fried notices something else moving deep in the water; as Zem plunges his arm in, an eerie submerged figure simultaneously reaches for the same object. The distance underwater is distorted, so that Zem must extend himself further then he thought to grasp the hammer. But at the exact same time the pale figure in the pool grasps his wrist, and pulls Zem firmly into the water! For Fried, the whole room seems to tilt and spin and flood with water; it's an inescapable whirlpool!

Swwwoosh! The pair of them are drawn helplessly down, down, down into the cold dark water. The room, the light and the objects are already far above them...

Freid (I hope) was able to get a big breath of air before being pulled under and down. He will try to stay with Zem and try to not panic.

Does it look like the light is coming from a particular direction here under the water? If it is that's the direction Freid will try to swim and drag Zem along if he can. He is reasoning that light means air and air is what they need!